In general terms, an ecosystem can be defined as an ecological unit consisting of a biotic community (an assemblage of plant, animal, and other living organisms) together with its abiotic environment (such as soil, precipitation, sunlight, temperature, slope of the land, etc.). The word ecosystem is an abbreviation of the term, "ecological system." A river, a swamp, a meadow, and a cave are all examples of ecosystems. Some consider the ecosystem to be the basic unit in ecology.
Humans are part of the environment and thus impact, and are impacted by, ecosystems. Forests provide wood for homes and an environment for recreation; wetlands purify our water; rivers provide fish and hydroelectric energy. Fifty percent of all jobs worldwide are tied to agriculture, forestry, and fishing. Human impacts often have caused dramatic changes to diverse ecosystems. Urbanization and industrial, agriculture, recreational, and forestry activities have impacted such things as biodiversity and numbers of organisms, modified biogeochemical cycles, and increased pollution.
The twentieth century exhibited humanity's ingenuity in many ways, including a history of intervening in major river and wetland systems by creating dams for hydroelectric plants or navigation, or by diverting water to open up wetlands for development. All large rivers in the temperate zone have been altered for human use, as have most of the world's large river floodplain ecosystems. It has been said that historically, if a forest, wetland, or river was not producing jobs and wealth, it was cut, drained, mined, or dammed. Clearly, the study of ecosystems and human impacts is important for creating a sustainable environment for future generations.
The size and scale of an ecosystem can vary widely. They may be very large, such as a tropical rain forest, the Everglades, or the Pantanal, or very small, such as a test tube of phytoplankton or an aquarium tank with plants and fish. Some even define a biome as an extensive ecosystem, although generally an ecosystem is viewed as having a more defined abiotic environment than a biome, and a biome as a group of ecosystems sharing broad environmental characteristics.
The boundary of an ecosystem is not always easy to delineate. Different ecosystems are often separated by geographical barriers, like deserts, mountains, or oceans, or are isolated otherwise, like lakes or rivers. As these borders are never rigid, ecosystems tend to blend into each other. For example, the boundary of a river may seem clear, yet caimans crawl from the river to bask in the sun, herons get food from the river but nest in trees, and tapirs may swim in the water and yet live on the land. To some extent, the whole earth can be seen as a single ecosystem, or a lake can be divided into several ecosystems, depending on the scale used.
Ecosystems may be categorized in different manners. Following are some examples of diverse ecosystems:
A major process linking the abiotic and biotic constituents of ecosystems is the flow of energy.
The main source of energy in almost all natural ecosystems is radiant energy from the sun. Primary producers or autotrophic organisms, such as plants, algae, and photosynthetic bacteria, take radiant energy and fix it into organic molecules by photosynthesis, such a creating glucose from carbon dioxide. Only a small portion of radiant energy actually is converted into biochemical form via photosynthesis. Studies suggest that ecosystems generally fix 3 percent or less of sunlight, and that for most ecosystems this figure is probably less than 1 percent. There are also other autotrophic organisms, such as chemosynthetic bacteria living around deep-sea vents that can manufacture their own food from chemical energy.
Energy then flows through the system when organisms eat each other. The trophic level, or feeding level, is a way of delineating the position of an organism in the food chain, that is, the relationship between what the organism eats and what it is eaten by. Autotrophs are at the base of food chains. Heterotrophs utilize the energy fixed in organic molecules by autotrophs. Herbivores, or primary consumers, are heterotrophs that eat autotrophs, such as antelopes that feed on grass or zooplankton that feed on phytoplankton in the ocean or in lakes. Carnivores are heterotrophs that eat herbivores or other carnivores, and include coyotes, tigers, owls, and preying mantises. Carnivores can be secondary consumers (those that eat an herbivore), or tertiary consumers (those that eat a carnivore that has eaten a herbivore), and so on. Omnivores are heterotrophs that consume either autotrophs (primary producers) or consumers (herbivores and carnivores), and include bears and humans. Scavengers, such as crows, are heterotrophs that feed on recently dead organisms. Decomposers are heterotrophs that obtain energy by breaking down dead organisms into their inorganic form, such as bracket fungi that break down dead tissues and wastes into carbon, nitrogen, and other inorganic compounds and elements. Autotrophs can then utilize these materials and use them in manufacturing food.
Energy flows through an ecosystem in the form of carbon-carbon bonds. As carbon-carbon bonds are broken, energy is released, which then can be used by the organism or dissipated as heat. Although energy flows through an ecosystem, only a portion of the energy available to an organism is actually stored by the organism, and thus the total energy in one trophic level never flows to the next level. That is, lower trophic levels always contain more total energy than higher trophic levels. Energy does not recycle, but ultimately all energy that is brought into an ecosystem is lost as heat.
A food chain identifies the sequence in which organisms obtain energy and feed in an ecosystem, such as from grass to insect to mouse to owl to scavenging vulture to decomposing bacteria. A food web shows a more complex relationship of feeding and energy flow among species in an ecosystem.
A second major process linking the biotic and abiotic constituents of an ecosystem is the flow of nutrients. Unlike energy, which is not cycled, inorganic nutrients are cycled in ecosystems. A biogeochemical cycle is the process by which inorganic materials, such as water, oxygen, carbon, calcium, and nitrogen, move through both the biotic communities (organisms) and the geological world (atmosphere, oceans, soil, and even rocks).
For example, in the nitrogen cycle, although about 78 percent of the atmosphere is nitrogen gas, most living organisms cannot use atmospheric nitrogen. There is a process that converts atmospheric nitrogen into compounds that plants can use, such as nitrites and nitrates. The nitrogen cycle includes four major processes. "Nitrogen fixation" is the process whereby bacteria convert nitrogen gas into ammonia compounds. The "nitrification process" involves chemosynthetic bacteria oxidizing ammonia compounds to produce nitrites and nitrates (which can also enter the soil from other sources, such as a bolt of lightning or erosion of certain rocks). Plants can utilize nitrites and nitrates to form amino acids. In the "ammonification process," bacteria break down nitrogen-contain amino acids from dead organisms or their wastes and form ammonia compounds (which, again, can cycle to plants via the nitrification process). In "denitrification," anaerobic bacteria break down nitrates, releasing nitrogen gas back into the atmosphere.
The term ecosystem first appeared in a 1935 publication by the British ecologist Arthur Tansley (Tansley 1935). However, the term had been coined already in 1930 by Tansley's colleague Roy Clapham, who was asked if he could think of a suitable word to denote the physical and biological components of an environment considered in relation to each other as a unit. Tansley expanded on the term in his later work, adding the ecotope concept to define the spatial context of ecosystems (Tansley 1939). Modern usage of the term derives from the work of Raymond Lindeman in his classic study of a Minnesota lake (Lindeman 1942). Lindeman's central concepts were that of functional organisation and ecological energy efficiency ratios. This approach is connected to ecological energetics and might also be thought of as environmental rationalism. It was subsequently applied by Howard T. Odum, sometimes called the “father” of ecosystems ecology, in founding the transdiscipline known as systems ecology.
Early conceptions of the ecosystem were as a structured functional unit in equilibrium. This equilibrium was characterized as above by how energy and matter flows between its constituent elements. Others considered this vision limited, and preferred to understand an ecosystem in terms of cybernetics. From this view, an ecological system is not a structured functional unit in equilibrium, but a functional organization at “dynamic equilibrium,” or what was also called “steady state.” The branch of ecology that gave rise to this view has become known as systems ecology. Steady state is understood as the phase of an ecological systems evolution when the organisms are "balanced" with each other and their environment. This balance is achieved through various types of interaction, such as predation, parasitism, mutualism, commensalism, competition, or amensalism. Introduction of new elements, whether abiotic or biotic, into an ecosystem tend to have a disruptive effect. In some cases, this can lead to ecological collapse and the death of many native species. The abstract notion of ecological health attempts to measure the robustness and recovery capacity for an ecosystem. That is, how far the ecosystem is away from steady state.
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